The Macintosh Toolbox implements many of the high-level features of the Classic Mac OS, including a set of application programming interfaces for software development on the platform.
There were two advantages to this mechanism: The system was further optimized by allotting some bits of the A-trap instruction to store parameters to the most common functions.
An alternative mechanism did exist, however, in the Code Fragment Manager, which was used to load and dynamically link native PowerPC programs.
The PowerPC system call facility, analogous to the A-trap mechanism, was used to interface with the Mac OS nanokernel, which offered few services directly useful to applications.
In conjunction with resources stored on the ROM chip, the Toolbox can turn the screen gray, show a dialog box with the signature "Welcome to Macintosh" greeting, and display the mouse cursor.
By using Toolbox to help boot the machine, a rudimentary Mac-like environment can be initialized before ever loading the System suitcase from disk (in fact before ROMs on NuBus cards were executed), which is when the decision to use 24-bit or 32-bit addressing has to be made.
Although the "Classic Mac OS" boot process is convoluted and largely undocumented, it is not more limited than an IBM PC compatible BIOS.
[4] This system was not used for PowerPC Linux, however, because Open Firmware in New World ROM machines requires a bootloader within an HFS filesystem—a reason having nothing to do with the Toolbox or "old-fashioned" Macs in general.
More narrowly, the Startup Disk control panel in the Classic Mac OS and later macOS only allows the user to select a mounted filesystem with very particular constraints.